Detergent compositions in tablet form are known in the art, as discussed below, and some products are now on the market. Tablets have several advantages over powdered products: they do not require measuring and are thus easier to handle and dispense into the washload, and they are more compact, hence facilitating more economical storage.
Detergent tablets are described, for example, in GB 911204 (Unilever), U.S. Pat. No. 3,953,350 (Kao), JP 60-015500A (Lion), JP 60-135497A (Lion) and JP 60-135498A (Lion); and are sold commercially in Spain.
Detergent tablets are generally made by compressing or compacting a detergent powder.
As pointed out in EP-A-522766 (Unilever), difficulty has been encountered in providing tablets which have adequate strength when dry, yet disperse and dissolve quickly when wet.
It has been found that highly compacted tablets which are strong enough to withstand packaging, shipping and handling without crumbling or producing dust do not disperse quickly in a wash liquor, and so do not give efficient cleaning. In some cases a lump of undissolved detergent may remain at the end of the wash cycle. On the other hand a lightly compacted tablet which shows fast dissolution in the wash liquor has poor handling characteristics, being breakable and also friable and producing dust.
There have been proposals to apply a coating to tablets. Thus, GB 983,243 and 989683 (Colgate) describe coating detergent tablets with an aqueous solution of polyvinyl alcohol and glycerol or other plasticisers, and then drying them with forced air, heated air or infra-red rays to harden the coating and evaporate the solvent.
These documents teach that the coating improves resistance to breakage and also gives resistance to abrasion.
EP-A-522766 teaches the use of a water soluble organic polymer as a binder material. This material, applied to the particulate composition before making into tablets, leads to an increase in strength of the tablets. It allows good tablet strength to be achieved, at moderate compaction pressure. The resulting tablets disintegrate quickly in water.
A comparative example in this document showed that the same material could merely be applied to the tablet after the compaction of the binder-free composition. However, the teaching of the document is that it is better to utilise the material as a binder within the tablet.